Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy and clean energy is increasing, and in a bid to meet the demand, the fields of electric power generation and electric energy storage using electric chemistry are most actively studied.
Secondary battery is a popular example of electrochemical devices utilizing such electrochemical energy, and its applications tend to expand gradually.
According to the shape of their cases, secondary batteries are classified into cylindrical batteries and prismatic batteries in which an electrode assembly is embedded in a cylindrical or rectangular metal can, and pouch-shaped batteries in which an electrode assembly is embedded in a pouch-shaped case constructed of an aluminum laminate sheet.
The electrode assembly embedded in the battery case is a chargeable and dischargeable power generation element formed of a lamination structure of a positive electrode/separator/negative electrode. The electrode assembly is classified into a jelly-roll type in which long-sheet type positive electrodes and long-sheet type negative electrodes coated with an active material are wound with separators interposed between the positive electrodes and the negative electrodes, and a stack type in which a plurality of positive electrodes and negative electrodes having a predetermined size are alternately stacked with interposed separators.
An advanced electrode assembly structure has been developed combining the jelly-roll type and the stack type into a stack/folding type electrode assembly formed by folding long continuous separator films along with a full cell of a predetermined unit size having a positive electrode/separator/negative electrode structure or a bicell of a predetermined unit size having a positive (or negative) electrode/separator/negative (or positive) electrode/separator/positive (or negative) electrode structure.
In addition, in order to improve the process performance of conventional stack type electrode assemblies and meet the demands for various types of secondary batteries, there has been developed a lamination/stack type electrode assembly having a structure formed by stacking unit cells in which electrodes are alternately stacked with separators into lamination.
On the other hand, the secondary battery repeatedly contracts and expands in the process of charging and discharging, which may generate a space between an electrode and a separator. With a space introduced between the electrode and the separator, lithium ions need to move further, increasing the internal resistance and deteriorating the overall performance of the secondary battery. Accordingly, there has been an attempt to use a separator provided with an adhesive coating layer having high tack strength in order to prevent the generation of space between the electrode and the separator.
This approach could increase an adhesion between the electrode and the separator of the secondary battery for improvement in the life expectancy and the high rate charge/discharge characteristics of the secondary battery. However, when applied to electrode assemblies requiring winding of the separator in the manufacturing process, the conventional method causes one end of the electrode assembly to be stuck to a winding core during their subsequent separation, which is called “tail out condition,” resulting in increased cases of defective appearance of the electrode assembly.
Therefore, there is a high demand for a technology capable of increasing the productivity of the electrode assembly while improving the life expectancy and high-rate charge/discharge characteristics of the secondary battery.